(1) Field of the Invention
The present invention relates to a method of optimizing the performance of an aircraft, to a device, and to an aircraft.
The invention is thus situated in the technical field of power plants for vehicles, in particular for an aircraft, and more particularly for a rotorcraft.
(2) Description of Related Art
An aircraft is usually provided with at least one engine for propelling it. For example, a rotary wing aircraft has a power plant provided with at least one engine for driving a rotary wing in rotation. By way of example, a helicopter is often fitted with at least one turbine engine, specifically a turboshaft engine, and sometimes known as a gas turbine.
Each engine is dimensioned to be capable of being used at at least one power rating, each power rating associating a developed power level with a duration of utilization.
Among known ratings, mention may be made of the following:
the take-off rating which associates a maximum take-off power PMD with a duration of utilization of about 5 minutes (min) to 10 min; and
the maximum continuous rating associating a maximum continuous power (PMC) with an unlimited utilization duration.
There also exist super-contingency ratings for aircraft having at least two engines, these ratings being for use when one of the engines fails:
a first contingency rating associates a super-contingency power with a duration of about thirty consecutive seconds known as 30 sec OEI (for one engine inoperative), this first contingency rating being usable on about three occasions during a flight;
a second contingency rating associating a maximum contingency power with a utilization duration of about two minutes, known as 2 min OEI; and
a third contingency rating associating an intermediate contingency power with a utilization duration extending to the end of a flight after one engine has failed, for example.
In parallel, it is common to determine the number of hours of flight that an aircraft engine can endure before it needs to be revised. This number of flight hours is known as the time between overhauls (TBO).
The performance of an aircraft then depends on the power that can be developed by each engine in the various power ratings. For example, the maximum take-off weight of an aircraft and its cruising speed are aspects of performance that depend in particular on the power developed by each engine.
Nevertheless, the power developed by an engine tends to decrease over time. The power levels developed by a new engine are generally higher than the power levels developed by an aging engine that is coming up to its Time between overhauls.
Under such circumstances, in order to guarantee the required performance independently of the age of an aircraft engine, it is possible to overdimension the engines.
It can be understood that the performance of a new engine and the performance of an aging engine may differ. Consequently, new engines may be more powerful than the engines actually required and certified for guaranteeing the performance of the aircraft throughout its lifetime. Each new engine thus presents a power margin over the required power levels.
Typically, an engine may present a power margin lying in the range 2% to 10% over the required and certified power levels. Under such conditions, the performance of an aircraft is thus guaranteed throughout the lifetime of its engines.
Nevertheless, it can be understood that each engine may present a power margin that is left unused in terms of aircraft performance.
Furthermore, certain certification regulations may require means for verifying that each engine is capable of delivering the power that enables the aircraft to reach the certified performance levels.
Consequently, it is possible to perform a health check on engines. The procedure for checking the health of aircraft engines serves to measure the operating margins of the engines for a given monitoring parameter.
For example, two monitoring parameters may be used to check the performances of an engine.
Since a turbine engine has a high-pressure turbine arranged upstream from a free turbine, a first monitoring parameter is the temperature of the gas at the inlet of the high-pressure turbine, known as TET by the person skilled in the art.
Nevertheless, since the temperature TET is difficult to measure because of its high value, it is preferable for the first monitoring parameter to be the temperature of the gas at the inlet of the free turbine, known as T45 by the person skilled in the art. This temperature is a good image of the temperature TET, and consequently it is representative of the degradation of the engine.
A first monitoring parameter is thus the temperature of a turbine assembly, this temperature possibly being the temperature TET of the gas at the inlet of the high-pressure turbine or the temperature T45 of the gas at the inlet of the free turbine.
Furthermore, another monitoring parameter relates to the power delivered by the engine or to the torque from its shaft, where power and shaft torque are mutually dependent. Nevertheless, the speed of rotation of the gas generator of the engine, known as Ng by the person skilled in the art, is also linked with the power delivered by the engine, so a second monitoring parameter that can be used is this speed of rotation Ng of the gas generator.
Consequently, checking the health state of the engine may consist, for example, in:
measuring the first monitoring parameter and then verifying that the current power value is greater than or equal to the power value that an aging engine would deliver under the same conditions; or
measuring the second monitoring parameter and then verifying that the current power value is greater than or equal to the power value that would be delivered by an aging engine under the same conditions.
By comparing the current value of a monitoring parameter with the minimum value that the monitoring parameter would have on an aging engine, the manufacturer can estimate the power margin of the engine.
It should be observed that it is also possible to measure information relating to the power developed by the engine for a given value of the first or the second monitoring parameter.
For example, on a rotary wing aircraft, measurements are taken of the torque developed by the engine and of the rotary speed Nr of the rotary wing. The power developed by the engine being monitored is then deduced therefrom in conventional manner.
Reference may be made to the literature in order to obtain information about the various procedures for checking the health of an engine.
Consequently, a manufacturer conventionally installs at least one overdimensioned engine on an aircraft in order to guarantee the performance levels of the aircraft between two overhauls. Furthermore, the manufacturer puts procedures into place for checking the health of the engine in order to verify that each engine can indeed develop power levels that enable it to ensure said performance levels.
This conservative approach is advantageous insofar as the performance of the aircraft is guaranteed. Nevertheless, during its lifetime, the aircraft may have available a margin of power of which no use is made in terms of performance.
In another approach, the maximum take-off weight of the aircraft is determined as a function of the power margin as determined during a health check of each of the engines of an aircraft. The maximum take-off weight is not set permanently by the manufacturer, but varies as a function of the results of health checks.
In a known implementation, this maximum take-off weight may vary in steps of 2% of a power margin, with the power margin being obtained as a result of a health check.
Determining the maximum take-off weight of an aircraft as a function of the available power margin is advantageous. Nevertheless, it is appropriate to check the health of the engines frequently.
Furthermore, a user may have difficulty in evaluating the utilization duration of an aircraft for a given maximum take-off weight, given that the maximum take-off weight is going to vary as a function of the wear of the engines.
The following documents are also known: FR 2 902 407; FR 2 899 640; U.S. Pat. No. 7,487,029; U.S. Pat. No. 8,068,997; and EP 1 741 901.